In order to impart flame retardancy to a thermoplastic or thermosetting resin, there has been adopted a method in which a flame-retardant agent is added in the process of molding the resin into an article. Examples of the flame-retardant agent include inorganic compounds, organophosphorus compounds, organohalogen compounds and halogen-containing organophosphorus compounds. Out of these compounds, organohalogen compounds and halogen-containing organophosphorus compounds exert an excellent flame-retardant effect. However, these halogen-containing compounds are pyrolyzed in the process of molding a resin to generate a hydrogen halide, which corrodes a metal mold, deteriorates the resin itself and causes coloration, degrading the working conditions. Another problem is that they generate a toxic gas such as a hydrogen halide, which is harmful to human bodies, when in a fire or incineration.
A halogen-free flame-retardant agent is therefore desired.
Examples of such a flame-retardant agent include inorganic compounds such as magnesium hydroxide and aluminum hydroxide; and nitrogen compounds such as melamine cyanurate, melamine phosphate and melamine polyphosphate. However, the inorganic compounds and the nitrogen compounds have a significantly low flame-retardant effect and therefore need to be added in a large amount to obtain a sufficient effect, leading to degradation of physical properties intrinsic to the resin.
As a flame-retardant agent that is halogen-free and provides a relatively good flame-retardant effect, there may be mentioned organophosphorus compounds, among which organophosphates are generally used. As a representative organophosphate, triphenyl phosphate (TPP) is well known. However, TPP is less heat-resistant and more volatile.
While recently developed high-performance plastics such as engineering plastics and super engineering plastics require a temperature as high as approximately 300° C. for molding, TPP is thus not proof against such a high temperature.
For a flame-retardant agent having thermo stability and low volatility, therefore, studies have been made on flame-retardant agents in a powdered state that are high-purity, well-moldable, and advantageous in terms of handling, packaging and transportation, focusing on aromatic diphosphates represented by the general formula (I) of the present invention (see Japanese Unexamined Patent Publication No. HEI 5(1993)-1079 (Patent Document 1) and Japanese Unexamined Patent Publication No. HEI 9 (1997)-87290 (Patent Document 2)).
In the preparation method disclosed in Patent Document 1, a high-purity aromatic diphosphate is obtained by recrystallization or crystallization using a solvent in a purification step after a reaction. Such a method requires the steps of solid-liquid separation, drying and recycling of the solvent and has a low yield due to loss from dissolution to the solvent, and therefore is not necessarily advantageous in terms of preparation steps and costs, assuming in particular a large industrial scale.
Patent Document 2 therefore has proposed a method in which an aromatic diphosphate obtained is solidified and powdered without being subjected to a special purification process.
According to the method of Patent Document 2, however, a phosphorus compound having a hydroxyphenyl group represented by the general formula (II)

wherein R1 and R2 each independently is an alkyl group having 1 to 5 carbons; R3 and R4 each independently is a hydrogen atom or an alkyl group having 1 to 5 carbons; Y is a bonding arm or a —CH2—, —C(CH3)2—, —S—, —SO2—, —O—, —CO— or —N═N— group; k is 0 or 1; and m is an integer of 0 to 4, is present as a by-product in the aromatic diphosphate due to the absence of a special purification process.
When the aromatic diphosphate including the by-product is added as a flame-retardant agent to a thermoplastic resin such as polycarbonate, it undergoes a transesterification reaction during a molding process, it reacts with an end of a molecule of the resin pyrolyzed, or it gradually exerts an adverse effect as a molded article of the resin is used for a long term, to reduce the molecular weight of the resin, and as a result, the durability, physical properties, water resistance, hydrolysis resistance and heat resistance of the molded article of the resin will be reduced.
In addition, the by-product undergoes transesterification with the aromatic diphosphate being a main component under a high-temperature condition such as in the molding process to cause further increase of the by-product, reducing the purity of the main component.
Japanese Unexamined Patent Publication No. 2003-160712(Patent Document 3) discloses a flame-retardant composition based on a phosphorus compound having a hydroxyphenyl group represented by the general formula (II) of the present invention. Patent Document 3 considers application of the phosphorus compound as a reactive flame-retardant agent for an epoxy resin by using the functional group (hydroxyphenyl group) of the compound of the general formula (II).
However, in additive flame-retardant agents based on aromatic diphosphates represented by the general formula (I) of the present invention, compounds having a hydroxyphenyl group represented by the general formula (II) of the present invention are not preferable, because they cause the above-described problems.